1. Field of the Invention
This invention relates generally to an apparatus and method for effecting the optical transmission of signals, and more particularly is directed to the optical transmission of a video signal comprised of a luminance signal and a carrier chrominance signal.
2. Description of the Related Art
An optical transmission system has been proposed for transmitting a luminance signal and a carrier chrominance signal without using wire. Such proposed system is comprised of an optical transmitter and an optical receiver to be hereinafter further described with reference to FIGS. 5 and 7, respectively, and which may be used, for example, for transmitting a video signal according to NTSC standards.
As shown in FIG. 5, the previously proposed optical transmitter 100 includes an input terminal 101 for receiving a luminance signal Y, a clamping circuit 102 receiving the luminance signal Y from the terminal 101 for making even the levels of the sink tip and the pedestal in the luminance signal, a pre-emphasis circuit 103 for emphasizing the high frequency band of the luminance signal received from the clamping circuit 102, and a frequency modulating (FM) circuit 104 for modulating the frequency of a carrier wave by the luminance signal Y which has had its high frequency band emphasized in the circuit 103. By making even the levels of the sink tip and the pedestal in the clamping circuit 102, a carrier wave can be modulated in the frequency modulating circuit 104 so that, for example, the sink tip of the luminance signal and the white peak of the luminance signal correspond to the frequencies 11.5 MHz and 13.5 MHz, respectively, in the frequency modulated luminance signal Y.sub.FM issuing from the frequency modulating circuit 104. The frequency modulated luminance signal Y.sub.FM is supplied from the modulating circuit 104 through a band-pass filter 105 for limiting the band thereof, for example, to a band of 6 to 20 MHz, to an amplifier 106, and the resulting amplified FM luminance signal Y.sub.FM is applied to a driving circuit 108 which correspondingly drives an infrared-emitting diode 107 constituting an electro-optical transducing means for outputting an optical signal corresponding to the FM luminance signal Y.sub.FM. The cathode of the diode 107 is shown connected to an output of the driving circuit 108, while the anode of the diode 107 is connected to a power terminal to which a suitable d.c. voltage V.sub.B is supplied.
The optical transmitter 100 is further shown on FIG. 5 to be provided with an input terminal 109 for receiving a carrier chrominance signal C having a chrominance subcarrier frequency of 3.58 MHz, and a frequency modulating circuit 110 for modulating the frequency of a carrier wave by the carrier chrominance signal C supplied to the terminal 109. In the case of the existing optical transmitter 100 being here described, a signal with the frequency of 25 MHZ is used as a carrier wave in the frequency modulating circuit 110. A band-pass filter 111 receives the FM chrominance signal C.sub.FM output from the frequency modulating circuit 110 for limiting the band thereof to 20 to 30 MHz. An amplifier 112 suitably amplifies the band-limited FM chrominance signal C.sub.FM prior to supplying the same to a driving circuit 114 for an infrared-emitting diode 113 which acts as an electro-optical transducing means for outputting an optical signal corresponding to the FM chrominance signal C.sub.FM. As shown, the cathode of the diode 113 is connected to an output of the driving circuit 114, while the anode of the diode 113 is connected to a power terminal to which a suitable d.c. voltage V.sub.B is supplied.
In the operation of the existing optical transmitter 100, a luminance signal Y supplied through the input terminal 101 has the levels of its sink tip and pedestal made even in the clamping circuit 102, and then has its high frequency band emphasized in the pre-emphasis circuit 103 prior to being frequency modulated in the circuit 104. The band of the FM luminance signal Y.sub.FM output from the frequency modulating circuit 104 is limited to 6 to 20 MHz by the band-pass filter 105 prior to it being amplified by the amplifier 106 and then supplied to the driving circuit 108 by which an infrared signal corresponding to the FM luminance signal Y.sub.FM is output from the diode 107 as a transmitted optical signal. A carrier chrominance signal C supplied through the input terminal 109 to the frequency modulating circuit 110 is there subjected to frequency modulation processing. The resulting FM chrominance signal C.sub.FM output from the circuit 110 is limited to the band of 20 to 30 MHz by the band-pass filter 111 prior to being amplified by the amplifier 112 and then supplied to the driving circuit 114 so that an infrared signal corresponding to the FM chrominance signal C.sub.FM is output from the diode 113 as a transmitted optical signal. The frequency bands of the optical signals transmitted from the diodes 107 and 113 of the optical transmitter 100 shown in FIG. 5, and which correspond to the FM luminance signal Y.sub.FM and the FM chrominance signal C.sub.FM, respectively, are shown in FIG. 6.
Referring now to FIG. 7, it will be seen that the existing optical receiver 200 suitable for receiving optical signals transmitted by the transmitter 100 includes a photodiode 201 serving as an electro-optical transducing means for transducing transmitted infrared or other optical signals to a corresponding electrical signal and a preamplifier 202 for amplifying an electrical signal output from the photodiode 201. The anode of the photodiode 201 is grounded through a coil 203 for providing bias, and the cathode of the photodiode 201 is connected to a power terminal to which a suitable d.c. voltage V.sub.B is supplied. A junction between the anode of the photodiode 201 and the coil 203 is connected to the input terminal of the preamplifier 202 through a capacitor 204 for reducing direct current. The optical receiver 200 further includes a band-pass filter 205 with a pass band of 6 to 20 MHz for extracting an FM luminance signal Y.sub.FM from an output of the preamplifier 202, a limiter 206 for limiting the amplitude of the FM luminance signal Y.sub.FM extracted by the band-pass filter 205, and an FM demodulating circuit 207 for demodulating the FM luminance signal Y.sub.FM output from the limiter 206. A low-pass filter 208 limits the band of a luminance signal Y output from the FM modulating circuit 207, whereupon, the high frequency band of such luminance signal Y is attenuated in a de-emphasis circuit 209 prior to the supplying of the luminance signal Y through an amplifier 210 to an output terminal 211.
The optical receiver 200 is further provided with a band-pass filter 212 also connected with the output of the preamplifier 202 and having a pass band of 20 to 30 MHz for extracting an FM chrominance signal C.sub.FM from the output of the preamplifier. Such extracted FM chrominance signal C.sub.FM has its amplitude limited in a limiter 213 and is then supplied to an FM demodulating circuit 214 for obtaining a carrier chrominance signal C. A band-pass filter 215 receives the chrominance signal C from the demodulating circuit 214 for limiting the band thereof prior to the supplying of such carrier chrominance signal through an amplifier 216 to an output terminal 217.
In transmitting a video signal according to the NTSC standard in which the frequency of a chrominance subcarrier is 3.58 MHz, if the transmission band of a carrier chrominance signal C is 1.5 MHz, the band required for normal double-sideband frequency modulation is 2.times.(3.58+1.50)=10.16 MHz. In transmitting a video signal according to the PAL standard in which the frequency of a chrominance subcarrier is 4.43 MHz, the band required for normal double-sideband frequency modulation is 2.times.(4.43+1.5)=11.86 MHz. Therefore, if frequency modulation is performed using a carrier wave of 25 MHz in the frequency modulating circuit 110 in the optical transmitter 100 of FIG. 5, the transmission band B.sub.N required for the NTSC standard extends from 19.92 MHz to 30.08 MHz, as shown on FIG. 8, and the transmission band B.sub.P required for the PAL standard extends from 19.07 MHz to 30.93 MHz, as also shown in FIG. 8. Therefore, the existing optical transmitter 100 described above with reference to FIG. 5 cannot be used with either the NTSC or the PAL standard for transmitting the FM chrominance signal C.sub.FM within the band of 20 to 30 MHz which is proposed for optical transmission of video signals by the Electronic Industries Association of Japan (EIAJ).
A further problem arises if the optical receiver 200 described with reference to FIG. 7 is employed for receiving the transmitted infrared or other optical signals from the transmitter 100. More specifically, in the receiver 200, the transmitted optical signals are transduced to an electric signal by the photodiode 201 and, after such electric signal is amplified by the preamplifier 202, an FM luminance signal Y.sub.FM and an FM chrominance signal C.sub.FM are extracted from the output of the preamplifier 202 by the band-pass filters 205 and 212, respectively. Since both the FM luminance signal Y.sub.FM and the FM chrominance signal C.sub.FM are frequency multiplexed or combined when amplified in the preamplifier 202, a secondary distortion of the FM luminance signal Y.sub.FM is generated within the band occupied by the FM chrominance signal C.sub.FM by reason of the non-linearity of the preamplifier 202, as shown in FIG. 9, and the FM chrominance signal C.sub.FM output from the band-pass filter 212 is undesirably influenced by such secondary distortion.